In a significant milestone in the battle against AIDS, Gilead Sciences (GILD) of Foster City on Thursday asked the government to make one of its drugs the world’s first approved medication to help prevent the spread of HIV, the virus that can lead to the deadly disease.

Studies of the drug have shown it sharply reduces the risk of HIV infection in those who don’t have the AIDS virus but who often engage in risky practices that lead to the disease. The drug, called Truvada, has been used for years as a treatment for people who already have HIV.

“This is really an exciting moment,” said Mitchell Warren, executive director of the AIDS Vaccine Advocacy Coalition, which closely tracks AIDS-related studies. “The implications are huge.” Although health authorities say the best way to avoid becoming infected with HIV is to use condoms and avoid other risky behavior, such as sharing drug needles, he said, “there are many people who can benefit from an additional prevention option.”

Because several international trials done by federal agencies and others have shown Truvada to be effective in blocking transmission of the virus, many people are hoping the U.S. Food and Drug Administration reviews Gilead’s application swiftly, said Dr. Connie Celum of the University of Washington, who conducted a study of the drug in Africa.

“People view this as a major advance,” she said of Gilead’s application, adding “there is a lot of enthusiasm that we now have biomedical tools that could help stem the number of new infections.”

It has been nearly three decades since federal health officials first identified HIV, the human immunodeficiency virus that causes AIDS. An estimated 33 million people have the virus worldwide and each year about 50,000 are infected with it in the United States, according to the federal Centers for Disease Control and Prevention.

Gilead did not conduct any of the studies where Truvada was used to block the virus, but it donated the drug and the placebos for tests.

“The data from these large-scale clinical trials suggest that Truvada may have a role to play in meeting the urgent public health need to reduce new HIV infections,” Gilead CEO John Martin said in a company statement. He added that, “we commend the many institutions, investigators and study volunteers for their commitment to advancing this important area of research.”

It remains uncertain how the FDA will respond to the request, and reviews of drug applications can take years. Given the need for a preventive treatment, advocates for those suffering from AIDS said they hoped Truvada proves worthy of approval. But they urged the federal agency to carefully review the data gathered from worldwide studies of the drug as an HIV blocker or so-called “pre-exposure prophylaxis.”

One study sponsored by the U.S. National Institutes of Health involved 2,499 high-risk HIV-negative adult men in the U.S., Africa, Asia and South America who have sex with men. Results published last year in The New England Journal of Medicine showed that the subjects who took Truvada reduced their risk of acquiring HIV by 44 percent compared with subjects taking a placebo. Those who used Truvada consistently — meaning they took it on at least 90 percent of the days they were on the study — cut their risk by 73 percent.

Similarly, in Celum’s study of 4,758 heterosexual couples in Kenya and Uganda, those taking Truvada once a day reduced their risk of getting HIV by 73 percent compared with those taking a placebo, according to preliminary data she presented at an international AIDS conference in July.

Yet another study by the Centers for Disease Control and Prevention of 1,200 HIV-negative heterosexual men and women in Botswana found that those taking the drug had 63 percent fewer HIV infections than those on the placebo. But an additional study of sub-Saharan Africa women was halted in April after a monitoring committee determined the test “would not be able to establish the efficacy of Truvada,” according to Gilead’s statement. It added that “the reason for this outcome is not yet understood and a complete detailed analysis of the data is currently under way.”

Previous tests of drugs deemed potential shields against HIV have flopped. And although some researchers have reported what they termed “modest success” with an HIV vaccine made from drugs developed by South San Francisco-based VaxGen and Sanofi Pasteur of France, others have criticized the data as being statistically meaningless.

If Gilead succeeds in getting Truvada approved as an HIV blocking agent, some analysts have speculated that it could mean hundreds of millions of dollars in additional annual sales for the company, whose revenue totaled nearly $8 billion last year. Following the announcement, which was made after the market’s close, Gilead’s shares rose 50 cents — or a little more than 1 percent — to $38.99.

“It’s a very important development,” Dana Van Gorder, executive director of Project Inform, an AIDS-advocacy group in San Francisco, said in response to Gilead’s application to make Truvada the first drug ever approved as a shield against the virus. “For people who are particularly at high risk, it may be a great solution to remain uninfected.”

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